Concentration-modulated absorption spectroscopy. Part 5.—Influence of molecular diffusion
The absorption of laser radiation by a chromophore creates a non-equilibrium distribution of excited-state species whose temporal decay back to the equilibrium distribution can be monitored by the familiar techniques of pump–probe spectroscopy. When this anisotropic distribution is created in a highly localised region at the focus of a laser beam and when the excited-state lifetime is suitably long, molecular diffusion provides an additional decay route for the recovery of the ground-state population in that region of space interrogated by the probe beam. The influence of this spatial decay in space and time on the population difference between the two electronic states coupled by the pump and probe laser beams is investigated theoretically and experimentally using the technique of concentration-modulated absorption spectroscopy. A modified form of the ‘gain’ expression of our earlier studies is proposed which is shown to account satisfactorily for the form of the ‘gain’vs. the logarithm of the modulation frequency curves observed experimentally. Although thermal lensing plays a part in influencing the transmission characteristics of the probe laser it is apparent that molecular diffusion is the dominant process in the experimental arrangement employed. The potential value of this technique for the study of molecular diffusion in highly localised regions of space is emphasised.